Over-current and short circuit protection circuits for protecting drive outputs, e.g., low-side drive outputs, are generally expensive and have a relatively wide tolerance. Unfortunately, protection circuits that have a relatively wide tolerance may allow for excessive power dissipation in the output driver that they are to protect. In general, automotive applications require protection of each connector pin of an assembly from short-to-battery faults and short-to-ground faults. A variety of techniques have been utilized to protect various input/output (IO) pins of an electronic device from faults. These techniques have included implementing self-protected output drivers, operational amplifier circuits and individual transistor circuits. However, these techniques tend to be relatively expensive and may not provide acceptable accuracy to prevent an output driver from experiencing excessive power dissipation.
With reference to FIG. 1, an electrical schematic of a relevant portion of a low-side drive circuit 100, which includes a current limit circuit 110, is shown. The circuit 110 acts as a current limiter for a low-side drive output and turns on when a voltage across the resistor Rsense exceeds the base-to-emitter voltage Vbe of transistor Q2. When the transistor Q2 turns on and the gate of transistor Q3 is pulled towards ground, the circuit 110 limits the current through the transistor Q3 (and the resistor Rsense) to maintain a voltage Vsense (at the base of the transistor Q2) to approximate the base-to-emitter voltage Vbe. Unfortunately, wide variations in the base-to-emitter voltage Vbe of the transistor Q2 with temperature causes a relatively wide variation in the current conducted by the transistor Q3.
Further, as the transistor Q2 operates the transistor Q3 in a linear mode, the transistor Q3 has a continuous relatively high power dissipation until a fault at the low-side of the load is removed. For example, assuming the resistor Rsense is a 200 milliohm resistor, the voltage at the low-side of the load is 15 volts and that the base-to-emitter voltage Vbe of the transistor Q2 varies between 0.4 and 0.7 volts, the transistor Q3 is current limited between 2 and 3.5 Amperes. This leads to a power dissipation for the transistor Q3 of between 30 and 50 Watts. It should be appreciated that requiring the transistor Q3 to dissipate at this power level until the fault is removed may shorten the life of the transistor Q3. Additionally, in the event that the power dissipation of the transistor Q3 reaches too high of a level, the transistor Q3 may fail.
What is needed is a relatively low-cost protection circuit that exhibits acceptable accuracy and limits power dissipation of a switch to an acceptable level.